A lithium ion secondary battery that is a typical example of a non-aqueous electrolyte secondary battery is characterized by having a high electromotive force and a high energy density while it is light weight. Therefore, demands for a lithium ion secondary battery as a driving power supply of various kinds of portable electronic equipment and mobile telecommunication equipment, for example, a portable telephone, a digital camera, a video camera, and a notebook-sized personal computer, have expanded.
A lithium ion secondary battery includes a positive electrode made of a lithium-containing composite oxide; a negative electrode including lithium metal, lithium alloy or negative electrode active material inserting and extracting lithium ion; and an electrolyte.
In recent year, instead of carbon materials such as graphite that have been conventionally used as a negative electrode material, studies on an element having a property for inserting a lithium ion and having a theoretical capacity density of more than 833 mAh/cm3 have been reported. An example of an element for a negative electrode active material, which has a theoretical capacity density of more than 833 mAh/cm3, may include silicon (Si), tin (Sn), germanium (Ge) to be alloyed with lithium, and oxide and alloys thereof. Especially, since silicon-containing particles such as Si particles and silicon oxide particles are cheap, they have been widely investigated.
However, these elements increase their volume when they insert lithium ions at the time of charge. For example, when the negative electrode active material is Si, the state in which the maximum amount of lithium ions is inserted is expressed by Li4.4Si. When Si is changed to Li4.4Si, the volume is increased by 4.12 times as the volume at the time of discharge.
Therefore, in particular, when a thin film of the above-mentioned elements is deposited on a current collector by a CVD method, a sputtering method, or the like, so as to form a negative electrode active material, due to the insertion/extraction of lithium ions, a negative electrode active material expands/contracts. During repetition of charge and discharge cycles, exfoliation may occur because of the deterioration in adhesion between a negative electrode active material and a negative electrode current collector.
In order to solve the above-mentioned problems, a method of providing convexities and concavities on the surface of a current collector, depositing a negative electrode active material thin film thereon, and forming gaps in the thickness direction by etching is disclosed (see, for example, patent document 1). Furthermore, a mesh is disposed above the current collector, and a negative electrode active material thin film is deposited through the mesh, thereby suppressing the deposition of the negative electrode active material in a region corresponding to the frame of the mesh is proposed (see, for example, patent document 2).
Furthermore, a method of providing convexities and concavities on a surface of a current collector and forming a film-like negative electrode material thereon in a way in which the negative electrode material is inclined with respect to the surface perpendicular to a main surface of the negative electrode material is proposed (see, for example, patent document 3).
In secondary batteries described in patent documents 1 and 2, a thin film of a negative electrode active material is formed as columnar bodies and gaps are provided between the columnar bodies, thus preventing exfoliation and wrinkles from occurring. However, since the negative electrode active material contracts at the time when charge is started, a metal surface of the current collector may be exposed through gaps. Thereby, since the exposed current collector faces the positive electrode at the time of charge, lithium metal tends to be precipitated, which may cause deterioration of safety and capacity. Furthermore, when the height of the columnar negative electrode active material is increased or the gap interval is reduced in order to increase the battery capacity, in particular, since the tip (opening end) of the columnar negative electrode active material is not regulated by the current collector and the like, it expands more as compared with the vicinity of the current collector as the charge proceeds. As a result, columnar negative electrode active materials are brought into contact with each other and pushed to each other in the vicinity of the tip portion. Thereby, the exfoliation between the current collector and the negative electrode active material and wrinkle of the current collector may occur. Therefore, it has not been possible to prevent exfoliation between the current collector and the negative electrode active material and wrinkles in the current collector from occurring and to increase the capacity simultaneously. Furthermore, since an electrolytic solution is trapped in the gaps between the columnar-shaped negative electrode active materials that have been expanded and brought into contact with each other, movement of lithium ions when discharge is started can be prevented. Therefore, in particular, there have been problems in discharge at a high rate (hereinafter, referred to as “high-rate discharge”) or discharge characteristics at a low temperature environment.
Furthermore, in a structure shown in patent document 3, as shown in FIG. 9A, with negative electrode active material 53 formed by inclining at an angle (θ), current collector 51 can be prevented from being exposed and precipitation of lithium metal can be prevented in advance. However, as in patent documents 1 and 2, as shown in FIG. 9B, negative electrode active material 53 expands larger as compared with the vicinity of current collector 51 as charge proceeds. Consequently, columnar negative electrode active materials are brought into contact with each other in the vicinity of the tip ends and pushed to each other as shown by arrows in FIG. 9B. As a result, exfoliation between current collector 51 and negative electrode active material 53 and wrinkles in current collector 51 may occur. Then, when negative electrode active materials are formed in a shape in which they expand but are not brought into contact with each other, on the contrary, high capacity cannot be realized. Furthermore, since an electrolytic solution is trapped in gaps 55 between columnar shaped negative electrode active materials that have been expanded and brought into contact with each other, the movement of lithium ions is prevented when discharge is started. In particular, there is a problem in discharge characteristics such as high-rate discharge, discharge in a low-temperature environment, and the like.    [patent document 1] Japanese Patent Application Unexamined Publication No. 2003-17040    [patent document 2] Japanese Patent Application Unexamined Publication No. 2002-279974    [patent document 3] Japanese Patent Application Unexamined Publication No. 2005-196970